1. Field of the Invention
This invention relates to a cover for a manhole or manway which provides access to an enclosed space, such as a tank, or other type of storage vessel. In particular, the invention relates to mechanisms for balancing the weight of the cover to assist in manual movement of same between open and closed positions.
2. Description of the Prior Art
Operating procedures for tank cars and other types of tank vessels commonly require that operating personnel not be required to lift in excess of a predetermined weight, e.g., 50 pounds. Hinged manway covers for such vessels typically weigh in excess of 100 pounds. Thus, the lifting force required at the handle to open the manway cover typically exceeds the allowable maximum lifting weight. Accordingly, it is known to provide such manway covers with balancing or lifting assisting mechanisms to provide a spring-biased assist to the operating personnel.
One such arrangement is made and sold by Midland Manufacturing Corp. and includes a pair of torsion springs mounted on opposite ends of the cover hinge pin. Each torsion spring has one end thereof received through an opening in the portion of the vessel wall defining the manway, and the other end thereof disposed through an eyebolt which is fixed through an opening in the manway cover. While this arrangement works well, it cannot be used with pressurized vessels because of the proscription against drilling holes in the cover or in the vessel wall which defines the manway. Standard operating procedures typically prohibit forming holes through or welding directly to a pressurized vessel wall.
Balancing mechanisms which are mounted entirely externally of the manway and the cover have been provided and are disclosed, for example, in U.S. Pat. Nos. 4,137,669 and 4,038,520. But both of these mechanisms are quite complex and consume a considerable amount of space, projecting a substantial distance radially from the manway.